Calculate the correct arity for async functions with destructuring - fixes #4977 (#4978)

packages/babel-helper-remap-async-to-generator/src/index.js

@@ -123,7 +123,7 @@ function plainFunction(path: NodePath, callId: Object) {
     REF: path.scope.generateUidIdentifier("ref"),
     FUNCTION: built,
     PARAMS: node.params.reduce((acc, param) => {
-      acc.done = acc.done || !t.isIdentifier(param);
+      acc.done = acc.done || t.isAssignmentPattern(param) || t.isRestElement(param);
 
       if (!acc.done) {
         acc.params.push(path.scope.generateUidIdentifier("x"));

packages/babel-plugin-transform-async-to-generator/test/fixtures/async-to-generator/function-arity/actual.js

@@ -1,4 +1,6 @@
 async function one(a, b = 1) {}
 async function two(a, b, ...c) {}
 async function three(a, b = 1, c, d = 3) {}
-async function four(a, b = 1, c, ...d) {}
+async function four(a, b = 1, c, ...d) {}
+async function five(a, {b}){}
+async function six(a, {b} = {}){}

packages/babel-plugin-transform-async-to-generator/test/fixtures/async-to-generator/function-arity/expected.js

@@ -28,4 +28,20 @@ let four = (() => {
   return function four(_x5) {
     return _ref4.apply(this, arguments);
   };
+})();
+
+let five = (() => {
+  var _ref5 = babelHelpers.asyncToGenerator(function* (a, { b }) {});
+
+  return function five(_x6, _x7) {
+    return _ref5.apply(this, arguments);
+  };
+})();
+
+let six = (() => {
+  var _ref6 = babelHelpers.asyncToGenerator(function* (a, { b } = {}) {});
+
+  return function six(_x8) {
+    return _ref6.apply(this, arguments);
+  };
 })();